1. Field of the Invention
The present invention relates to high-voltage variable resistors and particularly to a high-voltage variable resistor for adjusting focus voltage, screen voltage, and other voltages in, for example, a television receiver or other electronic apparatus.
2. Description of the Related Art
High-voltage variable resistors are generally used for adjusting focus voltages and screen voltages in television receivers. An example of known high-voltage variable resistors of this type is illustrated in FIGS. 6 to 9 (see also Japanese Unexamined Patent Application Publication No. 2000-200708). FIG. 6 is a front view of a high-voltage variable resistor, FIG. 7 is a bottom view of the high-voltage variable resistor, and FIG. 8 is a bottom view of the high-voltage variable resistor before being filled with insulating resin. FIGS. 9A and 9B are a side view and a bottom view, respectively, of a molded unit disposed on the rear surface of a circuit board.
This known high-voltage variable resistor 50 includes a case 51 made of insulating resin such as polybutylene terephthalate (PBT), a circuit board 52 disposed in the case 51, and a molded unit 53.
The case 51 has an opening at one end through which the circuit board 52 is mounted. A plurality of sliders (not shown, 6 sliders in this example) for adjusting resistance are disposed between the inner surface of the case 51 and the circuit board 52. Further, control shafts 54 for controlling the sliders extend through the upper portion of the case 51. The front surface of the circuit board 52 is provided with a circuit pattern (not shown) including resistive elements that are in contact with contacts of the sliders, and a plurality of electrodes electrically connected to external connection terminals 55 described below.
The molded unit 53 is made of insulating resin such as polyphenylene oxide (PPO) and PBT that are relatively inexpensive and have good moldability. The molded unit 53, parts of which are integrally molded, includes a capacitor housing 53a for accommodating a plurality of capacitors 56 (3 capacitors in this example), cylindrical terminal guides 53b for accommodating a plurality of external connection terminals 55 (9 connectors in this example), and connectors 53c for connecting the capacitor housing 53a and the terminal guides 53b. The capacitor housing 53a has resin inlets 53d. In the molded unit 53, the connectors 53c and each of the end portions of the capacitor housing 53a and the terminal guides 53b, the end portions being adjacent to the circuit board 52, are buried in the epoxy insulating resin 57, and thereby integrated with the case 51 together with the circuit board 52. The insulating resin 57 is poured over the entire rear surface of the circuit board 52 and cured.
In each external connection terminal 55, the lower end protrudes downward from each terminal guide 53b, while the upper end extends via a through hole of the circuit board 52 to a predetermined electrode in the circuit pattern on the front surface of the circuit board 52 and is soldered.
FIG. 10 is a circuit diagram of this known high-voltage variable resistor. In the drawing, VR1 to VR3 denote resistive elements for adjusting focus voltages, and VR4 to VR6 denote resistive elements for adjusting screen voltages. Further, IN1 denotes an input terminal for direct-current focus-voltage signals, IN2 denotes an input terminal for alternating-current parabolic-voltage signals for focus correction, OUT1 to OUT3 denote output terminals for focus-adjusting signals on which the parabolic voltage signals are superimposed, OUT4 to OUT6 denote output terminals for screen-adjusting signals, and GND denotes a ground terminal. Each of capacitors C1 to C3 serves as a coupling capacitor for superimposing the parabolic voltage signals on the focus voltage signals. For ease of understanding, the terminals shown in FIG. 10 and their electrically corresponding external connection terminals 55 shown in FIG. 7 are given the same reference numerals.
To assemble the high-voltage variable resistor 50, first the capacitors 56 are placed in the capacitor housing 53a of the molded unit 53, the external connection terminals 55 are inserted into the respective terminal guides 53b, and then lead terminals 56a for each capacitor 56 are connected to the respective predetermined external connection terminals 55.
Next, the molded unit 53 is attached to the circuit board 52 from the opening side of the case 51. The upper ends of the external connection terminals 55 are inserted into the through holes of the circuit board 52 to reach the electrodes on the upper surface, and are soldered.
Subsequently, the circuit board 52 is inserted from the opening of the case 51. The insulating resin 57 is then poured over the entire rear surface of the circuit board 52 and cured so that the molded unit 53 is secured to the case 51 together with the circuit board 52. Further, insulating resin is poured from the resin inlets 53d into the capacitor housing 53a and cured.
The known high-voltage variable resistor illustrated in FIGS. 6 to 9 has the following problems.
In a thermal shock test for assuring product quality, high-voltage variable resistors of this type are rapidly heated and cooled according to a predetermined heat cycle.
In the known high-voltage variable resistor, the entire rear surface of the circuit board 52 is filled with the insulating resin 57 to secure the molded unit 53 to the case 51. Since the material used to form the case 51 is different from the insulating resin 57, their coefficients of linear expansion are different. This causes an increase in thermal shock when the entire high-voltage variable resistor 50 is heated and cooled in the above-described test. As a result, the filled insulating resin 57 is peeled off the case 51, and the case 51 or the insulating resin 57 is cracked.
Moreover, pouring of insulating resin from the resin inlets 53d into the capacitor housing 53a is required in addition to pouring over the entire rear surface of the circuit board 52 to secure the molded unit 53 to the case 51. This causes an increase in usage of the insulating resin 57, man-hours, and costs.